1. Field of the Invention
This invention relates generally to low voltage dimmers and, more particularly, to use of a microprocessor to prevent burn-out of a low voltage transformer. If the load on the secondary side of the low voltage transformer is open, the time constant is increased. This increase in time constant is determined by the computer, which after one half cycle shuts the entire circuit down to allow the load to be re-established and power to be re-applied.
2. Description of the Prior Art
A typical low voltage light dimming circuit utilizes a primary and secondary winding of a transformer. The voltage is stepped down from the primary winding to the secondary winding, which then connects across the load, such as a light bulb. If the light bulb burns out, a DC voltage may be applied to the primary side of the transformer which would cause high current flow through the transformer. The high current flow will cause high temperatures in the transformer and possibly burn up the transformer. The typical solution in the past has been to fuse the circuit so that if high current flow exists, the fuse will blow. Another alternative is to install temperature shut-down devices. If the transformer gets too hot, the temperature switch will activate to open the circuit. Most manufacturers of low voltage dimmers specify that the loads to be used with the dimmers must be equipped with some form of protection such as a fuse or a temperature control device.
Generally it is well known to use the firing of a triac to control the amount of current being delivered to a load. A triac may be located either on the primary or secondary side of a low voltage transformer. Also, it is well known to use a solid state control circuit to control the phase angle at which the triac is fired. The earlier in each half cycle the triac is fired, the more power delivered to the load. If the load is a light, the brighter the light, the earlier the firing of the triac.
Concerning high voltage transformers, microprocessors have been used in the past to monitor a large number of parameters to determine what is occurring in the transformer. A typical such on-line power transformer is shown in U.S. Pat. No. 4,654,806 to Poyser. In Poyser, the microprocessor is used to collect numerous parameters concerning the transformer, but there is no indication of sensing time intervals between certain events such as the zero crossing and the next firing of a triac. Basically, Poyser simply teaches the use of the microprocessor to monitor transformers in a power line application. This is not the same type of application as low voltage light dimming.
Probably the best art known by the Applicant is U.S. Pat. No. 4,287,468 by Sherman, which is illustrative of a sizable body of art using microprocessors and dimmer circuits. Sherman shows the use of a microprocessor that counts the number of cycles of power being applied to the load as a part of the dimming technique. Also, a circuitry to check dimmer status is provided. However, Sherman does not show the use of a microprocessor to determine the time between a zero crossing and firing of a triac, and if it has not fired in a time interval, to fire the triac for that half cycle. Thereafter, current flow is removed until the open secondary transformer has been reconnected.
Generally it is well known to use a microprocessor to calculate time delays in various types of electrical devices as is shown in U.S. Pat. No. 4,275,445 to Di Pietro. However, none of the prior art devices known to Applicant are used to measure the time delay in firing of a triac based upon the increased impedance caused by an open secondary transformer that should, but does not, have a load thereacross.